AIR COOLING FOR HIGH TEMPERATURE 

POWER ELECTRONICS

Scot Waye, Matthew Musselman, Charlie King

National Renewable Energy Laboratory

2014 Thermal Management Systems Symposium

September 22–24, 2014

Denver, Colorado

14TMSS-0022

NREL/PR-5400-62920
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Project Objective

2

• Enable air-cooling thermal management of automotive power electronics,

with a focus on high-temperature devices

o Design, optimize, build, and demonstrate an air-cooled inverter

o Compare to conventional cooling systems: thermal resistance, coefficientof-performance, volume, weight

Motivation

• Help industry achieve the DOE APEEM 2015 30-kW (55-kW peak) inverter

targets (12 kW/L, 12 kW/kg) through improved thermal management

• Provide pathways to meeting 2020 targets (13.4 kW/L, 14.1 kW/kg)

Cost

Power Density, Specific Power, Efficiency
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Project Perspective

3

All vehicles are air cooled 

Air cooling a goal 

for batteries:

• Simple

• Direct

Just indirectly

Jim Snyder, NREL Image Gallery 15165
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State of the Art

4

LS 600H 2008

Camry 2007

Indirect Air Cooling via

Liquid Cooling

Intermediate liquid cooling loop 

rejects heat to air

Direct Air Cooling

Has been done before

Honda Insight (12 kW)

AC Propulsion (150 kW)

Mitsubishi SiC Inverter 

(50 kVA/L, 400 V output, 156 kVA)

SiC = Silicon Carbide

Low Power

Low Power Density

Prototype:

not in vehicle

NREL: John Rugh

Courtesy ORNL

DOE Advanced Vehicle Testing Activity 

(Idaho National Laboratory)
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Air as a Coolant

5

• Air is a poor heat-transfer fluid (low specific heat, density, conductivity)

• Large volume of air is needed

• Large surface area increases heat transfer, but increases volume

• Potential parasitic power of fan

• Not yet used in production vehicles

• Filtering, fouling, fan noise

Challenges

Advantages

• Rejecting heat directly to air can eliminate intermediate liquid-air loops

• Attractive for high-temperature device (wide-bandgap) applications

• Air can be used to cool other components (e.g., capacitors)

• Air is benign, is not carried, and does not need to be replaced

• Air is a dielectric and can contact the chip directly

• No global warming potential

• No liquid leaks
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System-Level Analysis

6
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High-Temperature Air-Cooled Inverter

7

• Electrical Design: Device type 

and location; electrical duty 

cycles; temperature-dependent 

losses, efficiency

• Thermal Constraints: Maximum 

junction temperature; heat 

generation; coolant temperature

• Feasibility / Trade-Off: Modeling; 

extrapolate to inverter scale

• Thermal Design: Sub-module 

testing and model validation; 

fan/ducting testing; optimization

• Thermal System Design: Balanceof-system analysis; full system 

models

• Hardware: Module prototype, 

improve design; balance-ofsystem testing

Thermal Constraints

Thermal System Design

Thermal Design

Feasibility / Trade-Off

Hardware

Electrical Design (ORNL) Design

Prototype

Development

Validation
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Module Optimization to Inverter Level Characterization

8

1Casing volume adjusted for fin geometry

1Capacitor ~1.13 La, ~1.62 kga

1Gate driver + control board ~0.88 La, ~0.42 kgb

aAssumption provided by ORNL; bNREL assumption based on similar device measurement

Heat dissipation for 55-kW peak power

From conservative analytical analysis ~ 2.7 kW heat (95% efficient)

• Fixed device heat generation and temperature (175oC)

• Parametrically optimized geometry by varying air flow 

rate using computational fluid dynamics (CFD) – ANSYS 

Fluent

• Extrapolated sub-module modeling and 

testing results to module level

• Added in balance-of-inverter components1

Baseline

Optimized

Capacitor Capacitor

2 devices

9 modules 6 modules
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Module Design Improvement (Parametric CFD)

9

Parametrically Changed:

• Device Location

• Fin Height

• Fin Length

• Base Plate Thickness

• Fin Thickness

• Channel Width

Improved weight 

and fan power

Improved volume 

and fan power

Modeling
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Optimized Geometries Improve Specific Power

10

Baseline

Optimized

Decreasing length

Increasing height

Changes to channel 

height and length

nine modules, DBC (direct-bond-copper)

Modeling
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Balance of System: Ducting

11

Example Production Vehicle Air Ducts

Production Duct 1 Production Duct 2

Volkswagen Touareg

Toyota Highlander

Toyota Camry

Toyota Prius

Nissan Altima

Hyundai Sonata

Chevy Volt

Inverter Location

Surrogate ducting 

pressure curves

Experimental
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Specific Power versus Parasitic Power for Various Heat 

Exchanger Geometries (Nine Modules)

12

Baseline

2015 

Target

2020 

Target

Power draw comparison

AC blower

AC condenser fan

Peak 

Steady state 

Low parasitic 

power but hard to 

meet targets

Modeling

nine modules, DBC (direct-bond-copper)
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Specific Power versus Parasitic Power for Various Heat 

Exchanger Geometries (Six Modules)

13

2015 

Target

2020 

Target

Power draw comparison

AC Blower

AC Condenser fan

Peak 

Steady-state 

Meeting targets 

with acceptable 

parasitic power

Decreasing height

Increasing length

Modeling

six modules, DBC (direct-bond-copper)
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Power Density versus Parasitic Power for Various Heat Exchanger 

Geometries (Six Modules)

14

2015 

Target

2020 

Target

Power draw comparison

AC Blower

AC Condenser fan

Peak 

Steady-state 

Meeting targets 

with acceptable 

parasitic power

Decreasing height

Increasing length

Modeling

nine modules, DBA (direct-bond-aluminum
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System-Level Test Bench

15

• Meets fan test standard

o ANSI/AMCA 210-07

o ANSI/ASHRAE 51-07

• Range: 5–500 cfm

• U95 Flow: ±1.5 cfm

• U95 Pressure: ±1.6 Pa

Fan pressure curves Fan efficiency curves

Fan or duct 

test subject

Blower

Experimental
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Example System Flow Study

16

Fan operation 45 – 70 W Heat

Pressure

2) Heat dissipation: 3.2 kW, 

exceeds 2.7 kW target

Example fan

3) Maximum flow 

rate: 115 cfm

1) Cross point of the 

fan curve and 

pressure loss
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Sub-Module Heat Exchanger Experiments

17

devices

Heaters

2.7-kW heat dissipation 

target for inverter

Baseline

Optimized

Prototypes provided by Sapa Extrusions USA (wire EDM)

Channel height – 15 mm

Channel length – 74 mm

Channel height – 21 mm

Channel length – 40 mm

ceramic heater Indium foil copper (with thermocouple) thermal epoxy

aluminum heat exchanger

Experimental
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Heat Dissipation for Baseline Case (Nine Modules)

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Specific Power*: 8.9 kW/kg

Power Density*: 10.1 kW/L

*does not include inverter casing

Extrapolated to 

inverter scale

Experimental
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Heat Dissipation for Baseline Case (Nine Modules)

19

~ 60 cfm

5 W Fluid Power

Heat dissipation: 2.7 kW target

Tout ~ 100oC

Specific Power*: 8.9 kW/kg

Power Density*: 10.1 kW/L

Fan sized to satisfy flow 

rate and pressure drop

*does not include inverter casing

Extrapolated to 

inverter scale

Experimental
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Heat Dissipation for Optimized Case (Six Modules)

Specific Power*: 14.3 kW/kg

Power Density*: 14.5 kW/L

*does not include inverter casing

Extrapolated to 

inverter scale

Experimental
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Heat Dissipation for Optimized Case (Six Modules)

Specific Power*: 14.3 kW/kg

Power Density*: 14.5 kW/L

~235 cfm

180 W Fluid Power

Heat dissipation: 2.7 kW target

Specific power and power density 

meet 2020 targets, with tradeoff 

of higher parasitic power

*does not include inverter casing

Extrapolated to 

inverter scale

Experimental
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Heat Dissipation for Optimized Case (Six Modules)

Specific Power*: 14.3 kW/kg

Power Density*: 14.5 kW/L

Heat dissipation: 2.7 kW target

Specific power and power density 

meet 2020 targets, with tradeoff 

of higher parasitic power

~135 cfm

40 W Fluid Power

*does not include inverter casing

Extrapolated to 

inverter scale

Experimental
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Heat Dissipation for Optimized Case (Six Modules)

Specific Power*: 14.3 kW/kg

Power Density*: 14.5 kW/L

Heat dissipation: 2.7 kW target

Specific power and power density 

meet 2020 targets, with tradeoff 

of higher parasitic power

~50 cfm

4 W Fluid Power

Tout ~ 120oC

*does not include inverter casing

Extrapolated to 

inverter scale

Experimental
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Inlet Temperature Effect (175oC Junction Temperature)

24

Experimental
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Fluid Power for Various Junction Temperatures

25

Experimental
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Inlet versus Outlet Temperature at Various Junction Temperatures

26

Experimental
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Module-Level Heat Exchanger Testing

ceramic resistance heaters

Experimental

27
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Heat Dissipation of Module-Scale Heat Exchanger

Experimental

Not extrapolated to 

inverter scale

28
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Heat Dissipation of Module-Scale Heat Exchanger

Flat curves show low sensitivity 

to flow rate (reaching asymptote)

Experimental

Not extrapolated to 

inverter scale

29
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Inlet and Outlet Temperatures of Module-Scale Heat Exchanger

Experimental

Not extrapolated to 

inverter scale

Outlet temperatures reasonable

30
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Summary

31

• Good potential technology for low cost, simple-tomanufacture options for high-temperature devices

• Eliminates liquid-to-air coolant loop

• Provides potential solution to cooling components with lower 

temperature limits (e.g., 85°C capacitor)

• Thermal performance for baseline and optimized sub-modules 

was characterized

• 10-kW module heat exchanger characterization and 

performance completed

• Parasitic power acceptable (in line with other vehicle 

components [e.g., A/C fan or condenser])
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Conclusions

32

• Reducing the number of modules (with penalty of 

increased heat generation from increased current 

and increased parasitic power) is beneficial for 

weight and volume considerations

• Further optimization to thermal and electrical 

solutions would further reduce weight and volume

• Balance of inverter component design and 

optimization vital to meeting targets

• More advanced heat exchanger designs may 

improve heat transfer efficiency with tradeoffs in 

manufacturability and cost

• Electrical and thermal aspects need co-design to 

meet targets